Electron discharge device



Oct. 4, 1960 w. F; NIKLAS ELECTRON DISCHARGE DEVICE Filed Feb. 16. 1959United States Patent M ELECTRON DISCHARGE DEVICE Wilfrid F. Niklas,Chicago, Ill., assignor to The Rauland Corporation, a corporation ofIllinois Filed Feb. 16, 1959, Ser. No. 793,431

l4 Claims. (Cl. 313-65) This invention relates to electron dischargedevices of the image converter type and more particularly to the objector pickup Window of an X-ray image converter envelope.

Conventional image converters are contained in an evacuated envelope,the body of which is substantially cylindrical in shape. One end sectionof the cylinder is vclosed and contains provisions for ophthalmicviewing of an image. The opposite end of the cylindrically shapedenvelope is closed and customarily includes a spherically shaped objector pickup window area of a material transparent to a particular incidentradiation, such as X-rays.

Within the radiationtransparent window area and in close proximity toit, or some instances actually applied on the inner surface of thewindow area, is a multiple layer pickup screen including a photoemissivecathode. 'I'he opposite end of the evacuated enclosure generallycontains a fluorescent viewing screen and interposed between thephotoemissive cathode and the iluorescent viewing screen is an electronoptical system for the purpose of focusing and accelerating an electronimage projected from the photoemissive cathode in response to theexcitation of incident radiation from a source or object image. It hasbeen noted that the pickup window must be transparent to the incidentradiationand in the 'case of X-ray image converters the window isgenerally composed of glass. Materials such as aluminum or beryllium mayalso be used but ditculties of procurement as well as applicationusually limit their use.

It is 4essential for the proper operation of an image converter that thesensitivity of the device be as high as possible and that the brightnessdistribution in the image be as true a reproduction of the X-rayattenuation in the object under observation as it is possible to obtain.Existing image converter devices using an X-ray transparent pickupwindow of uniform thickness throughout suffer from a deficiency in bothof the above elements due principally to the variation in transparencyof the uniformly radiation-transparent window. Because of the divergentcharacter of X-rays, it can beV seen that the X-ray path length througha uniformly thick pickup window, measured at the axis, is substantiallyless than the X-ray path length through the glass measured at any pointwhich is off the axis and the diiference in path length increases withthe distance from the axis. This change in path length becomesincreasingly pronounced when `the pickup window is hemispherical inshape which is the common practice in image converters of the X-raytype.

The transparent window introduces a loss or attenuation to the incidentradiation that may be assumed to be constant per unit length in thematerial and, consequently, with a divergent beam the loss experiencedis a maximum at the periphery of the window. The loss in X-raytransmission in the peripheral area of a uniformly thick pickup windowin response to a divergent X-ray beam is manifest in an unwantedreduction in brightness in the peripheral area of the reproduced image.

2,955,219 Patented Oct. v4, 1960 It is, therefore, a principal object ofthis invention to provide a new and improved image converter envelope inwhich one or more of the aforementioned difficulties encountered withprior art devices are eitheralleviated or substantially eliminated. y

It is another object of the kinvention to provide an image converterenvelope structure having a pickup win',- dow in which X-raytransparency is substantially improved.

Itis still Ia further objectl of the invention to provide an imageconverter envelope having a structure which Y permits large image areasto be viewed without suffering a deterioration in sensitivity orundesirable contrast differences.

An image converter envelope, constructed in accordance with the presentinvention, comprises a substantially cylindrical section defining atransmission path for an electron image. One end of the cylindricalsection is enclosed and contains provisions for viewing an image. Theopposite end of the cylindrical section is likewise closed and includesa window area of a radiation-transparent material having a thicknesswhich varies inversely with the distance measured radially from the axisof the cylindrical section and thereby provides a constant transmissionpath length for incident rays issuing from an external source along theaxis of the cylindrically shaped section.

The features of the present invention which are believed to be novel areset forth with, particularity in the ap- `van evacuated envelope havinga cylindrically shaped central section, fa s'huldervportion atf one endof the cylindrical portion yextendingradially inwardly toward the axisthereof and including provisions for viewing an image, and a'closureportion at lthe opposite end of the cylindrical section including anX-ray transparent window portion. `Morefspeciiically, there is anenvelope section 10 which, for most of its length, is substantially acylinder vand this section of the envelope is closed at one end by are-entrant press 11. The aforementioned vshoulder lportion of theenvelope is that Ywhich interconnects the cylindrical section with there-entrant press. The =re mainder of. the glass envelope comprises asubstantially spherical glass section 12 having a diameter approximatelylequal to that of the cylindrical portion of envelope section 10. Thesections 10 and 12 are pre-sealed around their perimeters to respectivemetal iianges 13a` and 13b which, in turn,` are sealed together byHeliarc welding or been separately processed. t

Suitably mounted' within envelope section 12 is an electron source inthe form of a large diameter multilayer pickup screen'14 whichapproximates a sector of a the `like after the two envelope sections 10and 12 have vsphere oriented so-that its concave surface faces toward.support member 14a. superimposed on the phosphor layer is the usualbarrier layer 14e which may be of Y aluminum oxide or the like,V and onthe surface of which is applied a photoemissive cathode layer 14d.Photoemissive cathode Y14a! may be of conventionalt antimonycesiumcomposition. f L

A small 'diameter fluorescent screenis positioned V'within the; envelopewithin the area encompassed by the'shoulder portion thereof and facingthe pickupk screen. More specically, re-entrant section 11 is closed bya flat glass plate 15,- having transverse dirmensionsY small withrespect .to those of the cathode and. bearing Aon its inner face asuitable uorescent coating'to constitute a viewing screen 15a. Silveractivated zinc-cadmium suliide or the like is thetiuorescent materialgenerally employed andY the screen is preferably aluminizedor Votherwiseprovided with arnetallic backing layerj 15b.

A focusing and accelerating electrode system is. interposed intermediatethe photoemissive cathode and viewing v screenV and it comprises aconductive focus. electrode 19 adjacent the inner'surface of theenvelope and an anode 16 encompassing viewing screen 15a. The focuselectrode spans'substantially'the entire cylindrical portion of theenvelope and may conveniently take the form of a conductive wall coatinghaving one end terminated at and electrically'connectedrto metalViiangesV 13a and 13b. The anode 16 is a metallic electrode structurewhich is partially cylindrical and partially conicalV in shape -terminatcd atrits large end with Va skirt portion Which may b e accommodatedby mounting over Vthe Ive-entrant, or returned portion of envelopesection-11.

The opposite end Vof theeanode terminates in a substantially 'spherical'shaped cap117- having anaxial and circular aperture 18 which -providesaccess for electrons originating at the photoemissive cathode, admittingsuch electrons for impingementV against viewing screen 15. Likefscreen15, the anode also has small transverse -dimensions relative to those ofthe cathode.Y This, it will ing screen having a much smaller diameterthan the photo'- cathode in order to obtain a'marked intensiication ofthe visible image. The convex face of anode cap 1 7-faces the concavesurface of; pickup screen'14. Theanode encloses viewing screen 15and-isY electrically connected thereto so that the screen is maintainedat the same electrical potential as theranode. lThe structural details.of the vimage converter'as thus far described may be entirelyconventional and will, be familiar to those skilled in the art'.,

In the operation of the. described de'vice,m`ultiple layer pickupscreenY14 is operated at or nearV ground potential Vthrough a lead extendingthrough envelope section 12 as indicated'at 24. The anode structureconnects through Y a'lead 25 to a high voltage Ysource (not shown).which establishes an anode potentialy of between and :35' kilovolts.Focusing potential is applied to electrode`19 by connection throughiiangesV 13a, V 13brand the'focus potential is usually in the orderof`several hundred volts.

VHaving established the Voperatingl potentials,` an X-ray Vimagernay beadmitted through X-.ray transparent .window be recognized, is usualpractice, employing a circular view- Y da=d cos are tan between thelX-ray source and X-ray transparent window 12V isobject 30 through whichtheV diverging' X-rays pass,` causing an X-ray image to'pass throughtransparent window 12 and impinge upon pickup screen 14'.V Aconversionis'here; accomplished in that'the 4X=r`ay image, by exciting pickupscreen 14, causes a corresponding electron image to be developed.Thisimage, in turn, ist accelerated along the'electron pathY of the tubeVtoimpin'ge upon screen 15 where a further-` imageY conversionrtakesplace,V

12m. i. .Cuge'alvisible imagejon this screen in the. usual lway.

. Wness is minimized. v

In prior art devices transparent window 12 is of uniform thicknessthroughout and the effective length of the X-ray path through the windowincreases with increasing radial spacing which results in an undesirablereduction in X-ray transparency which is most pronounced at theperipheral area of the window. The reduction in X-ray transparency withthe distance from Ythe axseof the image converter results in acorresponding reduction in X-ray intensity on pickup screen 14 andproduces an undesirable brightness variation in thek image reproducedonuorescent screen 15a. in particular, the brightness. of this image fallsaway or decreases in a manner which is essentially the same as thechange in effect-ive X-ray transparency of the transparent window due tothe efective increase'in X-ray path length with radial distance from thetube axis. it can be shown that for a hemispherically shaped pickupWindow with a radius of 8 inches and an actual uniform window thicknessof .080V inch and with a point X-ray source at an axial distance of 24inchesY outside thewindow, the edective thickness of the windowincreases rapidly to a degree Where it may suer in excess of 20 percentloss of X-ray transmission value within a radial spacing of 5 inchesfrom the axis of the system.l It can likewise be shown that similartransmissionV losses occur across the area of a dat window as occur inthe case of a hemispher-ical window;

The improvementsrcontributed by the present inventionceuter particularlyabout a preferred form of window 12 whichovercomes many deficiencies ofprior art structures and imparts to the device under consideration-verydesirable-properties. In accordancewith the invention, the thickness ofVwindow 12 varies inversely Vwith distance measured radially from axisa-a of cylindrical portion 1t). in order toV provide a constant.transmission path length Vfor any incident rays issuing from an externalsource X disposed along axis a--a. The thickness of the window variessuch that the divergent X-raysV originating from source X see equalglass thickness across the entire effective window diameter. Thus thephysical thickness of the window decreases with increasing distance fromthe axis of the window. The preferred window coniiguration canbeexpressed by the following geometric function:

Vda=actualV window Vthickness d=window thickness at-the axis of the tubeR=radius of curvature of outer surface of Window r=radial spacing of thepoint of incidence for X-rays from the axis of system s L=distance fromthe X-ray source to outer window surface along axis. Y

A Vpickup window constructed in the manner of this invention satisfiesthe foregoing equation and provides an approximately constanttransmission value for all incident X-rays. Thus, the brightnessdistortion experienced with prior structures, featuring a wiudowofuniform thick- While a particular embodiment of the invention has beenshown and described, it is apparent that various .changes andmodifications maybe made, and it is therefore intended in the followingclaims to cover all such `modificationsand changes as may fall withinvthe trueV yspirit andi scope of this invention.`

I claim:

1. An envelope for an image converter comprising: a substantiallycylindrical section defining a transmission path for an electron image;an end section aixed to and closing one end of said cylindrical sectionand having provisions for viewing an image; another end section axed toand closing the opposite end of said cylindrical section including awindow portion of a material which is transparent to invisibleradiations, which has the configuration of a sector of a sphere, andwhich has a thickness varying inversely with distance measured radiallyfrom the axis of said cylinder to provide a constant transmission pathlength for'incident rays issuing from an external source disposed onsaid axis.

2. An envelope for an image converter comprising: a substantiallycylindrical section delining a transmission path for an electron image;an end section aixed to and enclosing one end of said cylindricalsection and having provisions for Viewing an image; another end sectionaixed to and closing the opposite end of said cylindrical sectionincluding a window portion of a material opaque to visible radiation buttransparent to invisible radiations, which has the configuration of asector of a sphere, and which has a thickness varying inversely withdistance measured radially from the axis of said cylinder to provide aconstant transmission path length for incident rays issuing from anexternal source disposed on said axis.

3. An envelope for an image converter comprising: a substantiallycylindrical section defining a transmission path for an electron image;an end section atiixed to and closing one end of said cylindricalsection and having provisions for viewing an image; another end sectionaiixed to and closing the opposite end of said cylindrical sectionincluding a window portion of glass which is transparent to X-rays,which has the configuration of a sector of a sphere, and which has athickness varying inversely with distance measured radially from theaxis of said cylinder to provide a constant transmission path length forincident rays issuing from an external source disposed on said axis.

any@ X da=d cos arc tan where dg=actual window thickness d=windowthickness at the axis of the tube R=radius of curvature of outer surfaceof window r=radial spacing of the point of incidence for X-rays from theaxis of system L=distance from the X-ray source to outer window surfacealong axis.

References Cited in the le of this patent UNITED STATES PATENTS2,060,977 De Boer et al Nov. 17, 1936 2,153,614 Coeterier et al. Apr.1l, 1939 2,198,479 Langmuir Apr. 23, 1940 2,247,409 Roper July l, 19412,641,723 Farnsworth June 9, 1953 2,807,737 Wright Sept. 24, 19572,926,239 Skellett Feb. 23, 1960 UNITED STATES PA'IEMPV omer CTIFTECORRECTINl Patent Noc 299551219 @otober 4W" i960 wurm@ e, Niklas It ishereby certified that error appears in the above numbered pat entrequiring correction and that the -sad Letters Patent shouldread ascorrected below.

Column 2,I line ZOX1 for Vsiconv reed f Silicone Signed and Sealed this29th day of May 19620 (SEAL) Attea:

DAVID L. LADD ERNEST w, swIDER ttesting Gfcer Commissioner of Patents

